You need to have the full Jessie repository in your
sources.list; you should have at least
deb http://ftp.us.debian.org/debian jessie main deb http://ftp.us.debian.org/debian jessie-updates main deb http://security.debian.org jessie/updates main
(you can add
contrib if you want, and the corresponding
deb-src lines if you want to be able to download source code for the packages too).
sources.list contains at least those three lines, then run
apt update apt install firewalld
jessie-updates repository only contains updates to Jessie, it doesn’t contain the packages which were part of the original Jessie release and haven't been updated since. See http://unix.stackexchange.com/q/210394/86440What is the difference between the "jessie" and "jessie-updates" distributions in /etc/apt/sources.list? for more information.